Radar-based rainfall data provide an estimation of rainfall distribution which is based on data from multiple weather radar towers located throughout the state. Bias-correction algorithms are then applied which use supplemental rain gauge data. These algorithms are needed to resolve discontinuities which may occur at the limits of coverage of individual radar sites. The rainfall data used by the Water Atlas were provided by the Southwest Florida Water Management District (SWFWMD) and the St. Johns River Water Management District (SJRWMD). The data can be visualized as a grid of estimated rainfall amounts, with each grid cell representing a 4 km2 area (2 km x 2km). The SWFWMD and SJRWMD provide monthly rainfall estimates for each grid cell within each district's geographic boundary, with a lag of about one month. Data are available for the period from 1995 until the present.
Rainfall data are received on a monthly basis and the individual grid cell values are copied into database tables. These tables are then used to calculate rainfall statistics for different time periods (monthly, annual, period-of-record) and for different geographic areas: single grid cells, basins, watersheds, and/or WBIDs (watershed-based land areas associated with regulatory activity under the Clean Water Act). Rainfall statistics are calculated and stored in the database so that they may be displayed graphically in charts and graphs and shown on choropleth maps. A choropleth map uses colors or patterns to represent ranges of values associated with a particular phenomenon.
The following calculations are made for each level of geography (grid cell, basin, watershed, WBID, e.g.):